1. Field of the Invention
The present invention relates generally to optical switches of the integrated optic type, and more particularly to a liquid crystal based integrated optic switch capable of redirecting optical beams of arbitrary polarization state in a planar waveguide geometry. Additionally, the invention relates to an optical cross-connect architecture made up of an array of the liquid crystal based integrated optic switches capable of interconnecting a large number of single- or multi-mode optical fiber channels.
2. Description of the Prior Art
Currently, the vast majority of optical cross-connect geometries employ either free-space propagation (see U.S. Pat. Nos. 5,960,132, 6,040,935, 6,097,518 and 6,097,859) or a network of channel waveguides (M. Kondo et al., xe2x80x9cIntegrated Optical Switch Matrix for Single-Mode Fiber Networksxe2x80x9d, IEEE Trans. Microwave Theory Tech., Vol. MTT-30, pp. 1747-1753 (1982); M. Okuno et al., xe2x80x9c8xc3x978 Optical Matrix Switch using Silica-Based Planar Lightwave Circuitsxe2x80x9d, IEICE Trans. Electron., Vol. E76-C(7), pp. 1215-1223 (1993); U.S. Pat. Nos. 4,988,157, 5,699,462 and 5,978,527) to route the optical beams to, between, and from an array of optical switch elements. To operate effectively, these architectures require either extremely precise two-dimensional alignment of the free-space switch elements with the optical beams, as well as with the input and output optical channels, or a complex network of optical channel waveguides which must be fabricated to very precise tolerances. Other architectures, such as some based on mirror-type switches (U.S. Pat. No. 4,828,362) or on optical gratings (U.S. Pat. No. 5,255,332), can either be sensitive to the polarization state of the optical radiation being switched or require the use of optical amplifiers to offset losses within the cross-connect. Due to these exacting fabrication and/or system requirements, current optical cross-connect architectures can be difficult to implement.
It is therefore an object of this invention to provide an integrated optic switch that is capable of efficiently redirecting optical beams of arbitrary polarization state in a simple planar waveguide geometry.
It is also the object of this invention to provide an optical cross-connect architecture based on these planar waveguide integrated optic switches which is capable of dynamically interconnecting a large number of single- or multi-mode optical fiber input and output channels with very low optical loss and doing so with stable fiber-to-cross-connect opto-mechanical bonds but without the use of any optical channel waveguides within the optical cross-connect.
According to the preferred embodiment of the present invention, an integrated optic switch is formed within a planar waveguide structure such as that disclosed in H. Kogelnik, xe2x80x9cAn Introduction to Integrated Opticsxe2x80x9d, IEEE Trans. Microwave Theory Tech., vol. MTT-23, pp. 2-16 (1975) by filling a narrow trench in the planar waveguide core layer with a liquid crystal material, P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, Clarendon Press-Oxford, pp. 1-18 (1993). The trench extends through nearly the full thickness of the core layer and is covered by the planar waveguide""s upper cladding layer. The planar waveguide is made to support at least one optical mode in the direction normal to the waveguide but contains no structure(s) to confine the optical radiation in the lateral direction.
Lateral collimation of a beam of optical radiation is maintained within the planar waveguide by making the lateral beam width much larger than the optical wavelength of the radiation. Within the switch, the collimated beam is made to impinge on the trench at a high angle of incidence, and the length of the trench is made long enough to extend beyond the beam on both sides. The liquid crystal and planar waveguide core materials are chosen so that their refractive indices are equal when the liquid crystal material is in its isotropic phase. Additionally, the liquid crystal material is chosen to have positive birefringence so that its isotropic refractive index is greater than its ordinary refractive index when the material is in its nematic phase. For the switch to operate in the cross or ON state, the liquid crystal material is maintained at a temperature a few degrees below the clearing point so that the liquid crystal is in its nematic phase.
When in the nematic phase, the liquid crystal director is aligned along the axis of the trench. For this orientation of the liquid crystal director, both polarizations of the incident optical radiation experience a liquid crystal index essentially equal to the ordinary refractive index which is lower than the refractive index of the planar waveguide core material. The incident angle of the collimated beam is made to be above the critical angle for this combination of planar waveguide core and liquid crystal indices so that all of the incident optical radiation will be reflected from the planar waveguide core/nematic liquid crystal interface.
To drive the switch into the through or OFF state, the temperature of the liquid crystal is raised to a temperature a few degrees above the clearing point so that the liquid crystal is in the isotropic phase. In the isotropic phase, the refractive index of the liquid crystal material matches the refractive index of the planar waveguide core so that there is no reflection from the planar waveguide core/isotropic liquid crystal interfaces, and all of the incident optical radiation is transmitted through the liquid crystal filled trench. The switch can therefore be operated in either the ON or OFF state simply by holding the liquid crystal material at a temperature a few degrees below or a few degrees above the clearing temperature.
Also according to this invention, an optical cross-connect is formed within a planar waveguide structure by fabricating a two-dimensional array (Nxc3x97N or Nxc3x97M) of the planar waveguide integrated optic switches along with input and output linear arrays of integrated optic beam collimators. The arrays of beam collimators are aligned with respect to the switch array so that a linear array of point-source optical inputs is transformed into N collimated beam inputs to the switch array and so that the N (or M) collimated beam outputs from the switch array are refocused to a linear array of point-source optical outputs. One edge of the planar waveguide structure is made to coincide with the linear array of optical inputs and a second edge is made to coincide with the linear array of optical outputs. Single- or multi-mode optical fibers can then serve as the optical inputs and outputs to the cross-connect by opto-mechanically bonding them to the input and output edges of the planar waveguide structure. Each fiber is bonded at a location such that its core is both aligned with the planar waveguide core layer and with the location of the focal point of the corresponding beam collimator.
The thickness of the planar waveguide core layer and the refractive index of the upper and lower cladding layers are chosen to maximize the optical coupling between the optical fibers and the planar waveguide. Within the planar waveguide, the optical radiation from each input is confined in the direction normal to the core layer but is allowed to spread out in the lateral direction until it reaches the corresponding collimator where it is transformed into a collimated beam. After traversing the switch array, the still collimated output beams are refocused onto the array of output fibers by the output collimator array. The cross-connect is operated by holding one switch in each row in the ON state to direct that beam into the desired output beam path. During operation, therefore, a total of N switches in the array are held in the ON state and all other switches are held in the OFF state.